1. Field of the Invention
The present invention relates to a communication device, more particularly to a crystal-less communication device and self-calibrated clock generation method.
2. Description of the Prior Art
All kinds of wireless communications in today's electronic products, for example mobile phones, Wireless Metropolitan Area Networks (WMAN), Wireless Local Area Network (WLAN), Satellite Positioning Systems (GPS), and Bluetooth, in addition to emphasize their ease of use and size of appearance, the performance of those cannot be ignored from the focus of research and development. There is no doubt that the base technologies in transmit and receive component are their key functions and play an importance key role successfully.
Existing communications system designs require a crystal to generate reference frequency for the whole system for correct frequency and timing synchronizations in transmitter and receiver. However, the area occupied and the cost from a crystal is extremely large, as shown in FIG. 1. When the communication devices 10 and 20 establish wireless communication, the transmitter and receiver ends need to ensure that the timing and frequency in both communication devices 10 and 20 are synchronized. In order to achieve this objective, there must be a reference clock immune from the external environment changes, like variances in the manufacturing process, operating voltage and temperature. In order to get a reference frequency away from external environment changes, the current wireless transceivers are using an oscillator 30 as a reference frequency. The oscillator 30 with the basic physical characteristics can be a stable frequency source. However, the oscillator 30 share its considerable size, also its hardware price and mounting procedure are very expensive.
Nowadays, existing crystal-less related technologies only achieve limited context about the oscillator. Those technologies can control the frequency error generated by oscillator in a certain range, but this range is only the processor related application can accept.
In the U.S. Pat. No. 6,219,797, A microprocessor has an ability to operate via an external crystal oscillator or be switched to operate in a low power mode via an internal ring oscillator. This patent has the frequency error |Δf|≧2.5% which is 625 times bigger than the communications system can tolerate range (40 ppm), so it does not apply to communications devices.
In the U.S. Pat. No. 6,219,797, an integrated crystal-less device generates an output signal with the frequency of the output signal dependent at least in part on a resistive element. The provided circuitry for providing compensation for the temperature coefficient of resistive element, the circuitry includes a bandgap reference and a resistive network. This patent has the frequency error |Δf|≧2.5% which is 625 times bigger than the communications system can tolerate range (40 ppm), so it also does not apply to communications devices.
Krishnakumar Sundaresan, et. Al., (IEEE J. Solid-State Circuits, vol. 41, no. 2, pp. 433-442) reports on the design and characterization of a process, temperature and supply compensation technique for a 7-MHz clock oscillator in a 0.25/spl mu/m, two-poly five-metal (2P5M) CMOS process. The paper has the frequency error |Δf|≧2.5% which is 625 times bigger than the communications system can tolerate range (40 ppm), so it is impossible to apply to communications devices.
In WBAN applications, we have ultra-low power requirements for WSNs because of the portable and long period body health monitor considerations. Although crystal-less based system can reduce large power consumption, it suffers a significant clock mismatch between the CPN and WSNs that could damage the system performance because of SCO and CFO. This clock mismatch is resulted from the PVT issue when the ring oscillator is used. The existing crystal-less oscillator that the programmers are designed in a manner that in the absence of quartz crystal as a reference frequency support, how to overcome the external environment parameters drift of the oscillator frequency error. And these were related to the design of the microprocessor-related systems and applications.
However, in a wired or wireless communication systems, to send and receive signals successfully, the frequency of error that transceivers can tolerate at both ends is very low, so only to control the error of oscillator still cannot achieve applications' communication. In order to deal with the clock mismatch issue between the CPN and WSNs, we need to estimate the value of the mismatch and then “tune” or “adjust” the ring oscillator toward the correct clock period. Therefore, to find other solutions in communication systems in the realization of a crystal-less technology must have another thinking.